domino theory

from WordNet (r) 3.0 (2006)

domino theory
    n 1: the political theory that if one nation comes under
         communist control then neighboring nations will also come
         under communist control

from The Collaborative International Dictionary of English v.0.48

Domino theory \Dom"i*no the"o*ry\
   A political theory current in the 1960's, according to which
   the conversion of one country in South Asia to communism will
   start a sequential process causing all Asian countries to
   convert to Communism.

   Note: The apparent assumption was that an Asian country
         politically aligned with the West was as politically
         unstable as a domino standing on edge. It was used by
         some as a justification for American involvement in the
         Vietnam war, 1964-1972.
         [PJC]